Vertical cavity surface emitting laser having strain reduced quantum wells

ABSTRACT

A VCSEL with nearly planar intracavity contact. A bottom DBR mirror is formed on a substrate. A first conduction layer region is formed on the bottom DBR mirror. An active layer, including quantum wells, is on the first conduction layer region. A trench is formed into the active layer region. The trench is formed in a wagon wheel configuration with spokes providing mechanical support for the active layer region. The trench is etched approximately to the first conduction layer region. Proton implants are provided in the wagon wheel and configured to render the spokes of the wagon wheel insulating. A nearly planar electrical contact is formed as an intracavity contact for connecting the bottom of the active region to a power supply. The nearly planar electrical contact is formed in and about the trench.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Utility application Ser. No.11/224,615 filed Sep. 12, 2005 titled Vertical Cavity Surface EmittingLaser Having Multiple Top-Side Contacts, which is a continuation in partof U.S. Utility application Ser. No. 11/222,433 titled Vertical CavitySurface Emitting Laser with Undoped Top Mirror filed Sep. 8, 2005, whichclaims priority to U.S. Provisional Application No. 60/615,413, titledVertical Cavity Surface Emitting Laser with Undoped Top Mirror filedOct. 1, 2004, which are incorporated herein by reference in theirentireties.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The invention generally relates to lasers. More specifically, theinvention relates to Vertical Cavity Surface Emitting Lasers (VCSELs).

2. Description of the Related Art

Lasers are commonly used in many modern components. One use that hasrecently become more common is the use of lasers in data networks.Lasers are used in many fiber optic communication systems to transmitdigital data on a network. In one exemplary configuration, a laser maybe modulated by digital data to produce an optical signal, includingperiods of light and dark output that represents a binary data stream.In actual practice, the lasers output a high optical output representingbinary highs and a lower power optical output representing binary lows.To obtain quick reaction time, the laser is constantly on, but variesfrom a high optical output to a lower optical output.

Optical networks have various advantages over other types of networkssuch as copper wire based networks. For example, many existing copperwire networks operate at near maximum possible data transmission ratesand at near maximum possible distances for copper wire technology. Onthe other hand, many existing optical networks exceed, both in datatransmission rate and distance, the maximums that are possible forcopper wire networks. That is, optical networks are able to reliablytransmit data at higher rates over further distances than is possiblewith copper wire networks.

One type of laser that is used in optical data transmission is aVertical Cavity Surface Emitting Laser (VCSEL). As its name implies, aVCSEL has a laser cavity that is sandwiched between and defined by twomirror stacks. A VCSEL is typically constructed on a semiconductor wafersuch as Gallium Arsenide (GaAs). The VCSEL includes a bottom mirrorconstructed on the semiconductor wafer. Typically, the bottom mirrorincludes a number of alternating high and low index of refractionlayers. As light passes from a layer of one index of refraction toanother, a portion of the light is reflected. By using a sufficientnumber of alternating layers, a high percentage of light can bereflected by the mirror.

An active region that includes a number of quantum wells is formed onthe bottom mirror. The active region forms a PN junction sandwichedbetween the bottom mirror and a top mirror, which are of oppositeconductivity type (i.e. a p-type mirror and an n-type mirror). Freecarriers in the form of holes and electrons are injected into thequantum wells when the PN junction is forward biased by an electricalcurrent. At a sufficiently high bias current the injected minoritycarriers form a population inversion in the quantum wells that producesoptical gain. Optical gain occurs when photons in the active regionstimulate the transition of electrons from the conduction band to thevalance band which produces additional photons in phase with thestimulating photons. When the optical gain is equal to the loss from thetwo mirrors and the cavity, laser oscillation occurs. The free carrierelectrons in the conduction band quantum well are stimulated by photonsto recombine with free carrier holes in the valence band quantum well.This process results in the stimulated emission of in phase photons,i.e. coherent light.

The active region may also include an oxide aperture formed using one ormore oxide layers formed in the top and/or bottom mirrors near theactive layer. The oxide aperture serves both to form an optical cavityand to direct the bias current through the central region of the cavitythat is formed.

A top mirror is formed on the active region. The top mirror is similarto the bottom mirror in that it generally comprises a number of layersthat alternate between a high index of refraction and a lower index ofrefraction. Generally, the top mirror has fewer mirror periods ofalternating high index and low index of refraction layers, to enhancelight emission from the top of the VCSEL.

Illustratively, the laser functions when a current is passed through thePN junction to inject free carriers into the active region.Recombination of the electrons from the conduction band quantum wells tothe valence band quantum wells results in photons that begin to travelin the laser cavity defined by the mirrors. The mirrors reflect thephotons back and forth. When the bias current is sufficient to produce apopulation inversion between the quantum well states at the wavelengthsupported by the cavity, optical gain is produced in the quantum wells.When the optical gain is equal to the cavity loss laser oscillationoccurs and the laser is said to be at threshold bias and the VCSELbegins to ‘lase’ as the optically coherent photons are emitted from thetop of the VCSEL.

The VCSEL is generally formed as a semiconductor diode. A diode isformed from a pn junction that includes a p-type material and an n-typematerial. In this example, p-type materials are semiconductor materials,such as Gallium Arsenide (GaAs) doped with a material such as carbonthat causes free holes, or positive charge carriers to be formed in thesemiconductor material. N-type materials are semiconductor materialssuch as GaAs doped with a material such as silicon to cause freeelectrons, or negative charge carriers, to be formed in thesemiconductor material. Generally, the top mirror is doped with p-typedopants where the bottom mirror is doped with n-type dopants to allowfor current flow to inject minority carrier electrons and holes into theactive region.

Doping the mirrors results in various difficulties in lasers designed toproduce longer wavelengths. For example, as wavelength of the emittedlight increases, free carrier absorption also increases in the dopedmirrors. Free carrier absorption is the absorption of photons bycarriers (i.e. electrons and holes) in semiconductors which occurs otherthan across the band gap. Specifically, free carrier absorption occursat other non-photon producing bands other than between the valence andthe conduction bands.

In particular, holes cause free carrier absorption. Trap relatedabsorption is also prevalent in n-type AlGaAs. This added mirror lossrequires higher optical gain in the quantum wells to achieve thresholdbias. To achieve higher gain a higher current (i.e. threshold current)must be passed through the VCSEL to cause the VCSEL to lase. The highermirror loss also lowers the efficiency of the VCSEL. This increases theinternal heating and limits the amount of power that a VCSEL can output.

Further, the restriction on doping level caused by absorption in the topmirror decreases the electrical conductivity of the VCSEL which causesresistive heating in the VCSEL which limits power output, and degradesreliability.

Additionally, ramps of material composition at the boundaries betweenlayers in the mirrors degrade thermal impedance and reflectivity. If theVCSEL is not able to conduct heat away from the active region, theoperating temperature of the VCSEL may rise. If the mirror layers have adegraded reflectivity, additional layers may need to be used resultingin increased impedance and further increased heating of the VCSEL.Excessive heating can damage the VCSEL or shorten the useful life of theVCSEL or degrade its performance.

In summary, various issues relate to VCSEL technologies, including freecarrier absorption, series resistance, reliability, manufacturability,thermal resistance, polarization stability, single mode operation,optical isolation from external reflections and high temperatureperformance.

While the current designs have been acceptable for shorter wavelengthVCSELs such as VCSELs emitting 850 nanometer (nm) wavelength light,longer wavelength VCSELs have been more difficult to achieve. Forexample, in long wavelength VCSELs, absorption coefficients are larger.Additionally, due to the longer wavelengths and resulting increase inVCSEL layer size, increased absorption occurs. Specifically, VCSELs havelayers that are optimized based on the wavelength at which the VCSEL isdesigned to lase. A quarter wavelength section in a 1310 nm VCSEL islonger than a quarter wavelength section in an 850 nm VCSEL. Thus, a1310 nm VCSEL has more material where free carriers can be absorbed thanan 850 nm VCSEL.

A 1310 nm VCSEL would be useful in telecommunication applications. Themarket entry point of lasers used in 10 Gigabit Ethernet applications is1310 nm. However, due to the thermal and optical characteristics andother difficulties described above of currently designed VCSELs, 1310 nmVCSELs have not currently been feasible.

BRIEF SUMMARY OF THE INVENTION

One embodiment includes a vertical cavity surface emitting laser(VCSEL). The VCSEL includes a bottom DBR mirror that is formed on asubstrate. A first conduction layer region is formed on the bottom DBRmirror. An active layer, including quantum wells, is on the firstconduction layer region. A trench is formed into the active layerregion. The trench is formed in a wagon wheel configuration with spokesproviding mechanical support for the active layer region. The trench isetched approximately to the first conduction layer region. Protonimplants are provided in the wagon wheel configured to render the wagonwheel spokes insulating. A nearly planar electrical contact is formed asan intracavity contact connecting the bottom of the active region to apower supply. The nearly planar electrical contact is formed in andabout the trench.

Another embodiment includes a method of fabricating a VCSEL. The methodincludes forming a bottom mirror on a substrate. Forming a bottom mirrorincludes forming alternating layers of materials with different indicesof refraction. A first conduction layer region is formed on the bottommirror. An active layer region that contains quantum wells is formed onthe first conduction layer region. A second conduction layer region onthe active layer region is formed. The second conduction layer region iscoupled to the active layer region. A top mirror is formed on the secondconduction layer region. A trench is formed into the active layerregion. The trench is formed in a wagon wheel configuration with spokesproviding mechanical support for the active layer region. The trench isetched approximately to the bottom conduction layer region. Protons areimplanted into the wagon wheel configured to render the wagon wheelspokes insulating. A nearly planar electrical contact is formed as anintracavity contact to connect the bottom of the active region to apower supply. The nearly planar electrical contact is formed in andabout the trench . . . .

These and other features of the present invention will become more fullyapparent from the following description and appended claims, or may belearned by the practice of the invention as set forth hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand features of the invention are obtained, a more particulardescription of the invention briefly described above will be rendered byreference to specific embodiments thereof which are illustrated in theappended drawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be consideredlimiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 illustrates a VCSEL with undoped mirrors and intracavitycontacts;

FIG. 2A illustrates an epitaxial structure that may processed throughvarious lithographic processes to form a VCSEL;

FIG. 2B illustrates a graph of absolute electric field strength in aVCSEL versus the layers in an epitaxial structure when the VCSELincludes a semiconductor top mirror;

FIG. 2C illustrates a graph of electric field strength in a VCSEL versusthe layers in an epitaxial structure when the VCSEL includes adielectric top mirror;

FIG. 3 illustrates a deep RIE etch;

FIG. 4 illustrates a dilute HF etch;

FIG. 5 illustrates a trench etch;

FIG. 6 illustrates metal deposition;

FIG. 7 illustrates a wet oxidation step;

FIG. 8A-8E illustrate various metal deposition steps; and

FIG. 9 illustrates a top view of a VCSEL.

DETAILED DESCRIPTION OF THE INVENTION

To address various problems in long wavelength VCSELS, embodimentsdescribed herein are optimized in one or more areas. For example,embodiments may incorporate minimized distances, periodic doping wherecarriers are placed predominately at the nulls of the optical electricfield, minimized low mobility materials, and/or ramps in materialcompositions that are long enough to use low doping when they are not atnulls and thin higher doped portions when they are at or near nulls.

One embodiment improves performance of VCSELs at higher wavelengths byreducing or eliminating doping in the mirrors to minimize free carrierabsorption and trap related absorption. Dual intracavity contacts can beconnected to the active region via conduction layers to provide currentto the active region for producing photons. Other layers of the VCSELstructure also use a method of periodic doping where dopants are moreheavily concentrated at locations where the electrical field will be ata minimum so as to reduce free carrier absorption.

With reference now FIG. 1 an illustrative embodiment includes a VCSEL100 with top mirror 102, a p-type intracavity contact 103, an n-typeintracavity contact 104 and a bottom mirror 108. The VCSEL is formedfrom an epitaxial structure that includes various layers ofsemiconductor materials that will be described in more detail below. Theepitiaxial structure goes through various etching, deposition and oxidegrowth stages in the VCSEL 100 formation.

The VCSEL 100 is formed on a substrate 106. The substrate 106, in thisexample, is a gallium arsenide (GaAs) substrate. In other embodiments,the substrate 106 may be other material such as other III Vsemiconductor materials.

The bottom mirror 108 is formed on the substrate 106. The bottom mirror108 is a distributed Bragg reflector (DBR) mirror that includes a numberof alternating layers of high and low index of refraction materials. Inthe example shown, the bottom mirror 108 includes alternating layers ofaluminum arsenide (AlAs) and GaAs. In this example, the bottom mirror108 is undoped.

An active region is formed on the bottom mirror 108 and is boundedlaterally by proton bombarded isolation implants 111 which will bediscussed in more detail below. The active region includes quantum wells112. The central region of the quantum wells 112 under the oxideaperture 124 may also be referred to as the optical gain region. Thiscentral region of the quantum wells 112 is the location where currentthrough the active region and the presence of injected free carriers,holes and electrons, causes population inversion and optical gain. Theseelectrons from the conduction band quantum well states combine with theholes in the valence band quantum well states to cause the emission ofphotons. An oxide 114 is grown in an oxide layer 119 (See FIG. 2A) neara top conduction layer region 123 above the quantum wells 112 to providean aperture 124 for lateral definition of the laser optical cavity andfor directing bias current to the central region of the VCSEL activeregion.

In the embodiment shown, the top conduction layer region 123 includes aperiodically doped top portion. In the example, the top conduction layerregion 123 is p-type. Two or three periodically doped conduction layers116 (FIG. 2A) are heavily doped at the optical electric field minima (asillustrated in FIGS. 2B and 2C) so as to provide a low resistancelateral current path for directing current from the p-type intracavitycontact 103 to the central region of the active region located under theoxide aperture 124. The conduction layers may be doped at between about5×10¹⁹/cm3 to 1×10²⁰/cm3 at some distance less than about 14 nm awayfrom the minimum of the optical electric field. In one embodiment, thereare two periodically doped conduction layers each having a sheetresistance of about 500 Ohms per square which are doped with an acceptorimpurity such as carbon. Using two conduction layers results in a totalsheet resistance of about 250 Ohms per square.

Another embodiment of the invention, as illustrated herein, uses threeperiodically doped conduction layers 116 (FIG. 2A) in the top portion ofthe top conduction layer region 123. Each of the periodically dopedconduction layers 116 has a sheet resistance of about 500 Ohms persquare. Using three conduction layers results in a total sheetresistance of about 167 Ohms. This provides a significant reduction inseries resistance which lowers power dissipation and improves VCSELperformance.

A lower conduction layer region 125 is similarly doped so as tofacilitate electrical connections between the active region and n-typeintracavity contact 104. The undoped bottom mirror 108 may be grown onsemi-insulating substrates. This further reduces free carrierabsorption, and further allows for improved reflectivity and thermalconductivity by eliminating ramps between the bottom mirror 108 layers.To make contact to the n side of the p-n junction of the active region,the lower conduction layer region 125 is grown between the bottom mirror108 and the quantum wells 112. In this case the n-type intracavitycontact 104 is made after etching to the lower conduction layer region125. Etching to the lower conduction layer region 125 includes using anetch which stops on or in the lower conduction layer region 125.

The intracavity contacts 103, 104 are also formed such that theintracavity contacts 103, 104 are connected to bond pads for connectingthe VCSEL 100 to a current source. The bond pads are better illustratedin FIG. 9. In the example shown, the p-type intracavity contacts 103extend up around the sides of the undoped top mirror 102 mesa structure.This provides a thermal path for dissipating thermal energy generated inthe active region including the quantum wells 112 and the top conductionlayer region 123.

The undoped top mirror 102 in this example is part of a mesa structure.In the example shown the undoped top mirror 102 may be for examplealternating layers of higher and lower index of refraction materials.For example, the top mirror may include alternating layers of GaAs andAluminum Gallium Arsenide (AlGaAs). One present embodiment includesAlGaAs layers that are 87.5% aluminum. Other embodiments may includeAlGaAs layers that are generally in the range of 70-100% aluminum.

The VCSEL 100 may be constructed using a process that uses a protectiveoxide 306 to seal the top mirror 102 during the fabrication process ofthe VCSEL 100. In one embodiment, the protective oxide 306 may be 3λ/8thick. Because the top mirror 102 is sealed from the high temperaturewet oxidation process used to form the oxide layer 114, an alternateconstruction including alternating layers of GaAs and AlAs may be usedfor the top mirror 102. Layers of GaAs and AlAs may be advantageous inimplementing embodiments with better thermal characteristics. Betterthermal characteristics result because binary materials such as GaAs andAlAs are more thermally conductive than alloy materials such as AlGaAs.The use of binary materials in the top mirror 102 also provides thegreatest refractive index difference between the mirror layers whichmakes it possible to achieve the desired top mirror reflectivity with aminimum number of layer pairs. The VCSEL 100 further includes an oxidedielectric 120 to protect portions of the VCSEL 100. In one exemplaryembodiment the oxide dielectric 120 has an optical thickness of λ/2 ofthe VCSEL wavelength to provide maximum optical reflectivity at the topexit surface of the VCSEL 100. The oxide dielectric 120 may includeportions that are formed as a part of the initial plasma oxide 302 (FIG.3).

Various alterations may be made to the VCSEL 100 within scope ofembodiments of the present invention. For example, in one embodiment,one of the conduction layers in the conduction layer regions 123, 125may be replaced with a tunnel junction. A tunnel junction generallycomprises a highly conductive diode. The highly conductive diodeincludes a thin heavily doped p layer of GaAs and a thin heavily dopedlayer of n-type GaAs. The tunnel junction layers are sufficientlyheavily doped such that the resulting diode conducts by tunneling at andaround zero bias. Low lateral sheet resistance is provided by theheavily doped n-type layer, which has much higher carrier mobility thanthe heavily doped p-type layer. For example, and with reference to thetop conduction layer region 123, the p-type intracavity contact 103 iscoupled to the heavily doped n-type layer of the tunnel diode using analloyed Au:Ge contact layer under the top metal layer that forms theintracavity bond pad. The heavily doped n and p-type layers are locatedat or near minimums of the optical electronic field to minimize freecarrier absorption. While a tunnel junction alone may not havesufficient electrical conductivity to carry the appropriate currentsneeded to the active region, using a large area tunnel junction incombination with a conduction layer may provide an adequate amount ofconduction. This may allow for the use of a less than optimal tunneljunction in the VCSEL 100.

Very highly conductive tunnel junctions may be difficult to form inVCSEL structures. This difficulty results in part due to the hightemperatures used when forming additional layers of the epitaxialstructure subsequent to formation of tunnel junctions. These hightemperatures cause a diffusion of the high concentration dopants used inthe heavily doped layers in the tunnel junction. This diffusion degradesthe tunnel junction characteristics. However, using a less than optimaltunnel junction in conjunction with a conduction layer may provide asuitable current path for directing current to the active region.Additionally, a highly doped optimal tunnel junction may be used if theupper mirror is deposited at a low temperature made possible by the lackof doping and conduction. The upper mirror may be, simply a dielectricstack.

While various layers of the epitaxial structure 105 have been describedas being formed on other layers, this does not require that the layersbe formed directly on the other layers. Rather, other intervening layersmay exist between a layer and the layer in which that layer is formed.

The Epitaxial Structure

One embodiment is fabricated from an epitaxial structure 101, whosefeatures were described above and described in more detail now, such asthat shown in FIGS. 2A, 2B and 2C. Referring specifically to FIG. 2A, across-sectional view of the epitaxial structure 101 is shown. While theepitaxial structure 101 is typically formed on a substrate from thebottom up, the epitaxial structure 101 will be described from the topdown for clarity.

The Top Mirror 102

The epitaxial structure 101 includes a top mirror 102. The top mirrormay be either an undoped AlGaAs semiconductor mirror or a dielectricmirror. FIG. 2B and Table 1 below illustrate the standing electricalfield in various portions of the epitaxial structure and the epitaxiallayer details respectively for an undoped AlGaAs top mirror.

TABLE 1 Structure for semiconductor top mirror design in order ofgrowth, bottom to top. Al In N P- Thickness Sb N Number CompositionComposition Doping*1e18/cm3 Doping*1e18/cm3 nm comp comp Loop loopsComment loop 30 Low Strain 1 0 0 0 79.12 0 0 Section of 0 0 0 0 119.87 00 Lower Mirror endloop loop 8 Quarter 1 0 0 0 109.89 0 0 Wave 0 0 0 093.65 0 0 Section of Lower Mirror With endloop AlAs/GaAs loop 2 Quarter0.875 0 0 0 107.74 0 0 Wave 0 0 0 0 93.65 0 0 Section With endloopReduced Al 0.875 0 0 0 107.74 0 0 0 0 0.1 0 59.72 0 0 loop 2 Lower 0 03.2 0 46.83 0 0 Conduction 0 0 0.1 0 140.48 0 0 Layers endloop 0 0 1 046.83 0 0 Top Lower Conduction Layer 0 0 0.2 0 109.35 0 0 0 0 0.2 075.43754789 0 0 0 0 0 0 49 0 0 loop 4 Migration Enhanced 0 0 0 0 7.1 EPISection 0 0.26 0 0 5 0.018 0.02 Quantum 0 0 0 0 3 0 0 Well endloop 0 0 00 30 0 0 0 0 0 1 24.5210728 0 0 0.25 0 0 0.5 21 0 0 AlGaAs For ElectronConfinement 0 0 0 0.1 107.2 0 0 0.2 loop 3 Begin Ramp Up 0.125 0 0 0.50.2960325 0 0 Digital Alloy 0 0 0 0.5 0.2960325 0 0 Composition 0 0.5 0endloop Ramp Up 0.125 0 0 0.5 1.50060024 0 0 0.166666667 0 0 0.61.00040016 0 0 0.25 0 0 0.6 2.00080032 0 0 0.285714286 0 0 0.80.857485851 0 0 0.333333333 0 0 0.8 1.143314469 0 0 0.375 0 0 0.91.00040016 0 0 0.428571429 0 0 0.9 1.286228777 0 0 0.5 0 0 1.11.714971703 0 0 0.571428571 0 0 1.5 1.714971703 0 0 0.625 0 0 21.286228777 0 0 0.666666667 0 0 2.1 1.00040016 0 0 0.714285714 0 0 2.21.143314469 0 0 0.75 0 0 2.4 0.857485851 0 0 0.833333333 0 0 2.52.00080032 0 0 loop 5 Digital Alloy 0.875 0 0 2.5 0.8719 0 0 For Oxide 10 0 2.5 1.863079019 0 0 endloop loop 5 Second Half 1 0 0 2 1.863079019 00 of Digital 0.875 0 0 2 0.871920981 0 0 Alloy For endloop Oxide0.833333333 0 0 3.5 1.50060024 0 0 Ramp Down 0.75 0 0 3.5 0.643114389 00 0.714285714 0 0 3.5 0.857485851 0 0 0.666666667 0 0 3.5 0.75030012 0 00.625 0 0 3.5 0.964671583 0 0 0.571428571 0 0 3.5 1.286228777 0 0 0.5 00 3.5 1.286228777 0 0 0.428571429 0 0 3.5 0.964671583 0 0 0.375 0 0 3.50.75030012 0 0 0.333333333 0 0 3.5 0.857485851 0 0 0.285714286 0 0 3.50.643114389 0 0 0.25 0 0 3.5 1.50060024 0 0 0.166666667 0 0 3.50.75030012 0 0 0.125 0 0 3.5 1.12545018 0 0 0 0 loop 2 0 0 0 3.5 0.3 0 0Digital Alloy 0.125 0 0 3.5 0.3 0 0 endloop 0.06 0 0 80 25 ConductionLayer loop 2 Spacer 0 0 0 0.1 162.3 0 0 0.06 0 0 80 25 Conductionendloop Layer 0.875 0 0 0 431 0 0 Layer to Stop Etch Using OpticalReflectanc on RIE 0 0 0 0 81.2 0 0 Phase Matching Spacer loop 21 TopMirror 0.875 0 0 0 107.7 0 0 0 0 0 0 93.65 0 0 endloop

FIG. 2C and Table 2 below illustrate the standing electrical field invarious portions of the epitaxial structure and the epitaxial structurelayer details respectively for a dielectric mirror. The top mirror 102is fabricated so as to have low loss characteristics. Specifically bynot including dopants in the top mirror 102, or by limiting the amountof doping in the top mirror 102, absorption of free carriers isminimized in the top mirror 102 structure.

TABLE 2 Structure for dielectric top mirror design in order of growth,bottom to top. Al In N P- Thickness Sb N Number Composition CompositionDoping*1e18/cm3 Doping*1e18/cm3 nm comp comp Loop loops Comment loop 30Low Strain 1 0 0 0 79.12 0 0 Section of 0 0 0 0 119.87 0 0 Lower Mirrorendloop loop 8 Quarter Wave 1 0 0 0 109.89 0 0 Section of 0 0 0 0 93.650 0 Lower Mirror With endloop AlAs/GaAs loop 2 Quarter 0.88 0 0 0 107.740 0 Wave 0 0 0 0 93.65 0 0 Section With endloop Reduced Al 0.88 0 0 0107.74 0 0 0 0 0.1 0 59.72 0 0 loop 2 Lower 0 0 3.2 0 46.83 0 0Conduction 0 0 0.1 0 140.48 0 0 Layers endloop 0 0 1 0 46.83 0 0 TopLower Conduction Layer 0 0 0.2 0 109.35 0 0 0 0 0.2 0 75.43755 0 0 0 0 00 49 0 0 loop 4 Migration 0 0 0 0 7.1 Enhanced EPI Section 0 0.26 0 0 50.02 0.02 Quantum 0 0 0 0 3 0 0 Well endloop 0 0 0 0 30 0 0 0 0 0 124.52107 0 0 0.25 0 0 0.5 21 0 0 Enhanced Electron Confinement UsingAlGaAs Here. 0 0 0 0.1 107.2 0 0 0.2 loop 3 Begin Ramp Up 0.13 0 0 0.50.296033 0 0 With Digital 0 0 0 0.5 0.296033 0 0 Alloy For 0 0.5 0endloop Lowest Composition 0.13 0 0 0.5 1.5006 0 0 Digital Alloy 0.17 00 0.6 1.0004 0 0 for Lowest 0.25 0 0 0.6 2.0008 0 0 Composition 0.29 0 00.8 0.857486 0 0 0.33 0 0 0.8 1.143314 0 0 0.38 0 0 0.9 1.0004 0 0 0.430 0 0.9 1.286229 0 0 0.5 0 0 1.1 1.714972 0 0 0.57 0 0 1.5 1.714972 0 00.63 0 0 2 1.286229 0 0 0.67 0 0 2.1 1.0004 0 0 0.71 0 0 2.2 1.143314 00 0.75 0 0 2.4 0.857486 0 0 0.83 0 0 2.5 2.0008 0 0 loop 5 Digital Alloy0.88 0 0 2.5 0.8719 0 0 For Oxide 1 0 0 2.5 1.863079 0 0 endloop loop 5Second Half 1 0 0 2 1.863079 0 0 of Digital 0.88 0 0 2 0.871921 0 0Alloy For endloop Oxide 0.83 0 0 3.5 1.5006 0 0 Beginning of Ramp Down0.75 0 0 3.5 0.643114 0 0 Digital Alloy 0.71 0 0 3.5 0.857486 0 0 0.67 00 3.5 0.7503 0 0 0.63 0 0 3.5 0.964672 0 0 0.57 0 0 3.5 1.286229 0 0 0.50 0 3.5 1.286229 0 0 0.43 0 0 3.5 0.964672 0 0 0.38 0 0 3.5 0.7503 0 00.33 0 0 3.5 0.857486 0 0 0.29 0 0 3.5 0.643114 0 0 0.25 0 0 3.5 1.50060 0 0.17 0 0 3.5 0.7503 0 0 0.13 0 0 3.5 1.12545 0 0 0 0 loop 2 0 0 03.5 0.3 0 0 0.13 0 0 3.5 0.3 0 0 endloop 0.06 0 0 80 25 loop 2 0 0 0 0.1162.3 0 0 Spacer 0.06 0 0 80 25 Conduction Layer endloop 0 0 0 0 81.2 00 Phase Matching Spacer loop 9 Top Mirror Quarter wave SiO2 Quarter waveTiO2 endloop

When the top mirror 102 is an AlGaAs semiconductor mirror, extensive useof GaAs can be used to conduct heat away from the active region,including the quantum wells 112, so as to enhance reliability and tomaximize peak power output. Below the top mirror 102 is a phase matchinglayer 150. The phase matching layer 150 matches the phase to the topmirror 102.

The top mirror 102 may further designed to be strain reduced asdescribed below in the description of the bottom mirror 108.

Etch Stop Layer 122

Below the phase matching layer 150 is an etch stop layer 122. The etchstop layer 122 is used as a thick enough layer so that a reactive ionetch can be stopped reliably in this layer using optical monitoring or atimed etch. Then a selective etchant such as hydrofluoric acid (HF)etches the remainder of 122 and does not etch the low Al material in thetop conduction layer 116 during photolithographic processing of theepitaxial structure 101. The etch stop layer is composed of a high Almaterial. A low Al material is used for the conduction layer 116immediately below the etch stop layer, such that the selective etchetches to the top of the conduction layer 116 allowing easy contact tothe conduction layer 116. A disappearing layer (multiple of half waves)is used for the etch stop layer 122. Specifically, the etch stop layer122 is of a thickness that is about a multiple of a half wavelength ofthe wavelength at which the VCSEL 100 emits light. The optimal number ofhalf waves to give adequate etch tolerances and give suitable modeexpansion for single mode behavior is one wave. As mentioned previously,a matching layer 150 is used to match the phase in the top mirror 102.

Top Conduction Layers 116

Below the etch stop layer 122 are three top conduction layers 116separated from each other by top spacer layers 152. The top conductionlayers 116 are periodically doped such that high doping occurs at nullsin the standing e-field as shown in FIGS. 2B and 2C. The top conductionlayers 116 provide electrical conduction paths for the p-side contactsin this example. Periodic doping and p intracavity contacts aredescribed in U.S. Pat. No. 6,064,683 which is incorporated herein byreference.

Small quantities of Al are used in the top conduction layers 116. Thiscauses the amphoteric dopant carbon to be on the As site making it anacceptor. Without Al it is difficult at high doping levels to ensure thecarbon will be on the As site and remain there through subsequent hightemperature growth. In addition, to achieve the high doping levels,repeated delta doping with periods during which the surface is group IIIrich are advantageous. The common carbon sources which can be used fordoping are CBr₄, CCl₄, and in general the mixed compoundsCBr_(x)Cl_(4-x) where x is an integer 0-4. In MBE the incorporationefficiency is higher for the compounds with more Br and lower for thecompounds with more Cl. As such, delta doping techniques are especiallyuseful with more Cl. However, methods to control the low doping regionsusing compounds which do not dope as efficiently are often moreconvenient depending on the specifics of the system. One embodimentallows the achievement of even greater than 1×10²⁰/cm³ doping in theconduction layers less than about 3×10¹⁷/cm³ and preferably about1×10¹⁷/cm³ in other layers. The resulting sheet resistance of the topconduction layers above the oxide should be 250 ohms/square>Rs>100ohms/square. This allows a resistance between 50 ohms and 150 ohmsdepending on other parameters and allows for sufficient pumping of thefundamental mode without excess free carrier absorption. To avoid freecarrier absorption the thickness of the top conduction layers 116 shouldbe about 25 nm. In any case, the top conduction layers should be lessthan 40 nm. The top conduction layers 116 preferably have a combinedsheet resistance of less than 220 ohms/square and greater than 100ohms/square. Preferably the sheet resistance is about 180 ohms/square.Three top conduction layers 116 are used in the present embodiment.

Oxidation Layer 119

Beneath the conductions layers 116 is an oxidation layer 119 surroundedby ramp layers 154. The oxidation layer 119 is a layer constructed frommaterials that can be oxidized during a lithographic process so as tocreate an oxide aperture such as the aperture 124 shown in FIG. 1. Theoxidation layer 119 can be made from digital alloys as shown in Table 1attached hereto. A symmetric arrangement about a center thicker layer ofAlAs facilitates a sharper point on the leading edge slightly reducingscattering.

One of the reliability related issues for long wavelength VCSELs relatesto the oxide thickness. To address this, the oxidation layer 119 shouldbe designed to be as thin as is reasonable from a processing point ofview, but thick enough to support a significant electrical overstresswithout failure. An oxide which is too thick causes dislocations, andcan cause extra oxidation during aging. If it is too thin, it does nothold up well to electrical overstress, and oxidizes at a reduced rate.One design goal is to place the compositional ramps 154, the oxidationlayer 119 and a top conduction layer 116 at one null as shown in FIGS.2B and 2C. To accomplish this, the oxidation layer 119 thickness shouldbe less than 300 Å such as is shown in Table 1.

As discussed above, the compositional ramps 154, the oxidation layer 119and a conduction layer 116 are placed at one null. This minimizesdistances, maximizes the portion of the material with a high mobility,and uses the nulls of the optical field (illustrated in FIGS. 2B and 2C)for most of the doping. This helps to reduce free-carrier absorption andoptical loss while minimizing resistance.

Ramp Layers 154

Al ramp layers 154 are adjacent to the oxidizing layer 119. Thisminimizes the thickness of the lower hole mobility AlGaAs required andmaximizes the use of higher mobility GaAs. As illustrated in FIGS. 2Band 2C, the same null is used for the oxide layer 119, the ramp layers154 and one conduction layer 116. This minimizes vertical conductiondistances. Placing the ramp layers 154 adjacent to the oxidizing layer119 on both sides keeps the oxidizing layer 119 well defined so thatvertical oxidation does not make the oxide, grown as described belowfrom the oxidizing layer 119, thicker than desired. The oxidizing layer119, ramp layers 154 and conduction layer 116 combination is placed atabout the second null above the quantum wells 112 for good reliability.

Table 1 also shows the discrete layers and digital alloys making up theAl compositional ramp layers 154. In one embodiment, ratios of fluxes of1:2:4 for three of both three Ga sources and three Al sources are used.Other combinations and compositions may also be used. For example twoeach of Ga and Al may be adequate. When forming the ramp layers 154during an MBE process, a convenient growth rate is 1 ml/sec for thehighest flux sources. This allows fine enough steps in composition tominimize series resistance. Simulators such as SimWindows available fromthe Optoelectronics Computing Systems Center at the University ofColorado, Boulder or R-Soft Lasermod available from RSoft Design Groupof Ossining, N.Y., may be helpful in optimizing the compositional anddoping profiles.

Quantum Wells 112

The quantum wells 112 shown may be InGaAsN quantum wells with optionalAntimony (Sb). One reliability problem stems from high stress in quantumwells. In 1310 nm VCSELs with quantum wells made from approximatelyIn_(0.35)Ga_(0.65)As_(0.99)N_(0.01), the stress in the quantum wells ishigh enough to contribute to the formation of dislocation related darkregions. Thus, in the present embodiment, nitrogen is increased to about2% such that the Indium can be reduced to about 28% to achieve the samewavelengths. This results in a quantum well that is approximatelyIn_(0.28)Ga_(0.74)As_(0.98)N_(0.02). This reduces the stress to a pointwhich discourages the formation of dislocation related dark regions.Unfortunately this is not without penalty. The effective density ofstates in the conduction band increases causing a higher transparencycurrent. Typically the photoluminescence efficiency also decreasessignificantly signifying less efficient luminescence and poorer materialquality. To counter this effect trace Sb can be used in place of some Inon a nearly one for one basis.

Sb acts as a surfactant keeping the surface flat preventingthree-dimensional growth. This allows for a much higher growthtemperature, about 80° C. hotter than with In alone, and allows theachievement of photoluminescence signals similar to those achieved atmuch lower nitrogen levels, showing the higher luminescence efficiency.The optimal composition for this embodiment is on the order ofIn_(0.26)Ga_(0.74)As_(0.96)Sb_(0.0018)N_(0.02) with significantvariation allowed. Similar procedures are described in Voltz, K. et. al.“The role of Sb in the MBE growth of (GaIn)(NAsSb)”, Journal of CrystalGrowth 251 (2003) 360-366 which is incorporated herein by reference inits entirety. For example, the quantum well may be grown using an Sbflux greater than 7×10⁻⁸ and an Sb flux less than 12×10⁻⁸. The issuewith this technique is the reduction of the conduction band well depthand enhancement of the valence band well depth. Using low Sbcompositions avoids this problem. In addition, added AlGaAs layers forelectrical confinement can be placed close to the active region toenhance electrical confinement. Further, because the density of statesis higher, and to further reduce the strain related reliability problem,the quantum well thickness can be reduced to around 50 Å. To compensatefor the reduced gain distance, four quantum wells 112 may be used asopposed to three quantum wells 112. In one embodiment the VCSEL includesInGaAsNSb wherein Antimony (Sb) is used in place of In on anapproximately one for one basis.

To achieve optimal luminescence efficiency and narrow photoluminescentlinewidths, three-dimensional growth should be avoided. If the growinginterface is flat, such as achieved using migration enhanced epitaxy, orvery low growth rates at a low V/III, the photoluminescence linewidth isreduced significantly. Migration enhanced epitaxy is performed beforeand between the quantum well growth shown in Table 1 attached hereto.Migration enhanced epitaxy is described in more detail in U.S. patentapplication Ser. No. 10/931,194 filed on Aug. 31, 2004 and in relatedapplications including application Ser. No. 09/217,223 filed Dec. 12,1998; Ser. No. 10/026,016 filed Dec. 20, 2001, Ser. No. 10/026,019 filedDec. 20, 2001, Ser. No. 10/026,044 filed Dec. 27, 2001 and Ser. No.10/026,020 filed Dec. 27, 2001. Each of the cited applications isincorporated herein in their entireties. Portions of the barrier layersor the entire barrier layer may also contain nitrogen for straincompensation. The barrier layers may also contain Sb to enhanceflattening.

Quantum wells that contain any amount of nitrogen can experience 3-Dgrowth, thereby causing broadening of the spectrum as well as enablingthe formation of quantum dots. By flattening the adjoining surface (e.g.typically the barrier layers) just before the growth of nitrogencontaining quantum wells, no seed is provided or made available for 3-Dgrowth within the quantum well layers of a device. Sometimes barrierlayers associated with a quantum well will contain nitrogen as well,therefore any non-nitrogen layer should be flattened prior to growth ofthe nitrogen containing layer whether the nitrogen-containing layer is abarrier layer or quantum well. he present inventors have found thatMigration Enhanced Epitaxy (MEE) can be provided as a solution forflattening surfaces, and thereby eliminating quantum dot production orother 3-dimensional growth effects. MEE can be used to flatten devicelayers before steps are taken to grow nitrogen containing quantum wells,or associated barrier layer, resulting in desirable long wavelengthspectra. In a device that uses nitrogen and any combination of In, Ga,As and Sb in the quantum wells and/or any combination of In, Ga, As, N,Sb and P in the barrier layers, using a flattening layer before thegrowth of any nitrogen containing layers can be very beneficial. UsingMEE, for example, before, and/or after and/or between anitrogen-containing quantum well has been shown by the present inventorsto flatten a surface such that there is no seed available for 3-Dgrowth. According to the present invention, the use of MEE for achievingflattening can be performed by alternately depositing single atomiclayers of group III constituents and group V constituents. In particularGa and As work well on a GaAs substrate.

High temperature performance is limited by a combination of effects. Athigh temperatures confinement of carriers to the quantum wells 112becomes marginal. This is helped by good thermal design to remove heatand minimizing series resistance to avoid excess heat generation. Inaddition, the depth of the quantum wells 112 is important to reducecarrier leakage past the quantum wells 112. For InGaAsN(Sb) containingquantum wells 112, nitrogen enhances the depth of the conduction bandquantum well significantly which reduces electron leakage. Sb on theother hand increases the valence band well, but decreases the conductionband well. In addition, doping spikes, and extra AlGaAs confining layersas shown in Tables 1 and 2 can be used to enhance confinement.

Lower Conduction Layers 117

FIG. 1 also shows the lower (n-type in this example) conduction layers117 in a lower conduction layer region 125 below the quantum wells 112.Because of difficulty with high doping, and made possible by the higherelectron mobility, the lower conduction layers 117 are both thicker andmore lightly doped than the top conduction layers 116. Table 1 shows thespecifics of the structure displayed in FIG. 1. This particular designis for a 1280 nm VCSEL.

Lower Mirror

Trap related absorption occurs in n doped AlGaAs such as Si dopedAlGaAs. The use of undoped lower mirrors addresses this issue.

A high degree of bow built up in the wafers occurs due to the very thickepitaxy which is not lattice matched precisely to GaAs. AlAs for examplehas a slightly greater lattice constant than GaAs, 5.6605 Å vs 5.6533 Å.The bow becomes a problem when the wafers are thinned causing a largeamount of breakage. To reduce this, the lower mirror 108 can be designedin an asymmetric fashion such that more than a quarter wave of GaAs isused in each pair of alternating layers and less than a quarter wave ofAlAs is used in each pair of alternating layers. Table 1 attached heretoillustrates this design. When the layer thicknesses are no longer aquarter wave, the optimal mirror design no longer needs to be preciselymade of half wave pairs, but instead is optimized for maximumreflectance at the lasing wavelength. Though the lower mirror 108 couldbe made entirely using this concept, because the thermal conductivity ofAlAs is higher than GaAs and because the reflectivity of a pair ofquarter waves has higher reflectivity than non quarter wave pairs, itbecomes desirable near the active region to use the normal quarter wavesof AlAs and GaAs. Adjacent the lower conduction layers to avoidpotential oxidation of the AlAs layers due to non flat etches it isadvantageous to use lower composition AlGaAs layers instead of AlAs suchas Al_(0.875)Ga_(0.125)As layers. The lower mirror strain reducedstructure is shown in Table 1 attached hereto.

Standing Electric Field in the Epitaxial Structure

Referring now to FIGS. 2B and 2C, graphs that show the optical electricfield versus the layers in the epitaxial structure are shown. FIGS. 2Band 2C are used to illustrate where dopants may be heavily concentratedor avoided to reduce free carrier absorption in the epitaxial structure101. The electric field illustrated in FIGS. 2B and 2C is shown as anabsolute (positive) value, whereas in reality, the electric field may bepositive or negative in various locations of the epitaxial structure.However, because free carrier absorption is a function of the electricfield squared, which is always positive, the absolute value of theelectric field is useful for designing the epitaxial structure.

FIGS. 2B and 2C illustrate the top mirror 102. The top mirror 102 isundoped so as to reduce free carrier absorption. Free carrier absorptionis a function of the level of doping. Thus by not including any dopantsin the top mirror 102, free carrier absorption in the top mirror 102 isreduced or avoided.

Below the top mirror 102 is an etch stop layer 122. The etch stop layer122 is used in fabricating the VCSL 100 (FIG. 1). This will be explainedin more detail in conjunction with the description of FIGS. 3 through 8Ebelow. The etch stop layer 122 can be used to ensure that etching isdone to expose the surface of the conduction layer region 123 and thatthe etching does not remove a thin heavy doped layer on the surface ofthe conduction layer 116. This insures that the p-type intracavitycontact 103 may be properly attached to the conduction layers 116 toform low resistance Ohmic contacts. The etch stop layer 122 may beundoped. The etch stop layer 122 in this example is a grown AlGaAs layerwith aluminum composition between 60% and 87.5%.

Periodic doping involves heavily doping a layer at locations where theelectric field will be at a minimum when the VCSEL 100 (FIG. 1) is inoperation. This helps to reduce free carrier absorption in a layer. Theperiodically doped conduction layers 116 in the top conduction layerregion 123 are GaAs. A stop etch is used to expose the surface of one ofthe conduction layers 116 that etches AlGaAs but does not etch GaAs. Theetch rate of the stop etch is a function of the aluminum compositionwhich can be optimized to result in the optimum process control.Periodic doping methods are discussed in U.S. Pat. No. 6,064,683 titledBandgap Isolated Light Emitter issued to Ralph Johnson, which isincorporated herein by reference.

Below the etch stop layer 122 are three conduction layers 116. Theconduction layers 116 also use periodic doping to heavily dope portionsof the conduction layers 116 where the electric field is at a minimumwhile controlling doping to a lower impurity concentration when theelectric field is not zero.

An AlGaAs layer 224 with, in one embodiment, 96% aluminum forms theoxide layer 114 during high temperature wet oxidation. This highaluminum AlGaAs layer 224 is formed by digital alloy growth. In thepresent embodiment, the digital alloy growth includes 10 layer pairs of0.87 nm 87.5% AlGaAs and 1.86 nm of AlAs to give a total layer thicknessof 27.3 nm with an average composition of 96% Al and 4% Ga in theAlGaAs. The first 5 layer pairs start with AlGaAs and end with AlAs. Thelast 5 layer pairs start with AlAs and end with AlGaAs. This gives acentral region of AlAs with a thickness of 3.72 nm. The rate ofoxidation of AlGaAs is a rapidly increasing function of Al composition.Thus, the central AlAs layer provides a sharp point on the oxidationfront which is beneficial in limiting optical scattering loss at theoxide aperture and improves VCSEL performance.

Epitaxial Structure Processing to Produce the VCSEL

Attention is now directed to various acts and processes for fabricatinga VCSEL. Referring now to FIGS. 3 through 8E, various masks used inlithography steps for manufacturing the VCSEL 100 (FIG. 1) on a GaAswafer are shown. The lithographic steps are performed on an epitaxialstructure so as to form an VCSEL structure such as the VCSEL 100structure shown in FIG. 1. Prior to applying any lithographic masksshown, an initial plasma oxide 302 is deposited on the epitaxialstructure 101. In this example, the plasma oxide deposition isapproximately 1100 Å of SiO₂. 1100 Å is λ/8 wavelength in a 1310 nmlaser.

As shown in FIG. 3, portions of the initial λ/8 oxide 302 are removed.The mask layer used to remove the portions of the initial oxide 302 mayalso used to form fiducial marks for subsequent mask alignments.Additionally, the initial plasma oxide 302 may be used to protect areasfrom the deep etch described below. The remaining portion of the initialλ/8 oxide 302 is over the aperture 124 of the VCSEL 100 (FIG. 1).

FIG. 3 further illustrates a deep reactive ion etch using a Cl basedsystem with an inductively coupled plasma (ICP-RIE). Photoresist 304 ispatterned, using lithography, onto the VCSEL using an RIE etch mask. TheRIE etch mask is designed to overlap the remaining portions of theinitial oxide 302 so as to protect the remaining SiO₂. After the RIEetch mask is applied, a reactive ion etch is performed to etch to theetch stop layer 122 (FIG. 2A). Etching to the etch stop layer 122 mayinclude etching into the etch stop layer 122. However, care should betaken to keep overetching into the etch stop layer 122 at a minimum. Inone embodiment, etching to the etch stop layer etches into the 87.5%layer shown in Table 1.

Referring now to FIG. 4, a dilute HF etch is then done to etch throughthe etch stop layer 122 to the top conduction layer 116. In thisexample, photoresist 304 is formed into a HF shield mask to protectsidewalls of the mesa including the top mirror 102. A dilute HF is 300parts deionized water to 1 part hydrofluoric acid. Other solution ratiosmay also be used. The dilute HF etch should be performed such that thetop mirror 102 is not undercut. A small shelf of material, as shown inFIG. 4, should be left.

The photoresist 304 from the RIE etch mask is then removed. Referringnow to FIG. 5, another 3λ/8 wavelength layer of SiO₂, protective oxide306, is then deposited conformally using plasma enhanced chemical vapordeposition (PECVD) such that there are portions of the VCSEL 100 with3λ/8 wavelength SiO₂. Other portions of the VCSEL 100 where the SiO₂ hasbeen previously deposited and not etched away, such as where the initialoxide 302 exists, now have λ/2 wavelength SiO₂. In particular, the VCSELaperture 124 (FIG. 1) now has λ/2 wavelength SiO₂ above it. Because the3λ/8 protective oxide 306 surrounds the undoped top mirror 102 (FIG. 1)at this point, the top mirror 102 (FIG. 1) can be alternating layers ofAlAs and GaAs. The SiO₂ protects the otherwise vulnerable AlAs from thewet oxidation step that would otherwise oxidize completely the AlAs.Binary compositions of AlAs and GaAs are used in one embodiment becausethey give a larger index difference and higher thermal conductivity.While this example illustrates using 3λ/8 protective oxide 306, theprotective oxide 306 may be a number of thicknesses that are (½n−⅛)λ,where n is an integer.

With continued attention directed to FIG. 5, a trench photomask is usedfor a plasma etch and an additional RIE etch that will be done to removematerial in the trenches to a depth at the lower (n-type in thisexample) conduction layer region 125. In this example, a trenchphotomask 356 forms a wagon wheel structure trench with 5 spokes. Awagon wheel structure trench includes alternating etched and non-etchedportions. The spoke configuration provides mechanical stability. Furthermechanical stability may also be provided when the p-type intracavitycontact 103 (FIG. 1) is formed completely around the trench as will bedescribed further herein. While a single spoke keyhole design example isillustrated here, it should be understood that other embodiments mayinclude a wagon wheel or other design. Additionally, the trench may beformed in an oval or other asymmetric pattern to facilitate polarizationcontrol as described below.

With reference now to FIG. 6, various masks and other details are shown.FIG. 6 illustrates for example forming the n-type intracavity contactmetal 104 (FIG. 1). An n-type metal mask is used to appropriately form alayer of photoresist 308 for a metal liftoff step. About 1000 Å of AuGeare deposited on the wafer where the VCSEL 100 is being formed. The AuGelayer 310 is deposited on the photoresist 308 and on the exposed lowerconduction layer region 125. The photoresist layer 308 is removed andthe n-type intracavity contact 104 remains as is illustrated in FIG. 7.

With reference now to FIG. 7, a wet oxidation step will be performed toform the oxide 114 to a depth of about 12 μm. A high temperature, wetaluminum oxide 114 is grown from the edge of the trench to form theaperture 124 for the VCSEL.

A stepper isolation pattern mask is used to define areas of the VCSEL100 that will be isolated from other VCSELs on a wafer by use of highenergy proton bombardment. Further, the stepper isolation pattern maskdefines regions of the VCSEL 100 that will be rendered non-conductive soas to prevent current flow and lasing in those areas. This helps todefine the active region in the VCSEL 100. Protons are then implanted inthe regions defined by the stepper isolation pattern mask. Damageproduced by the protons renders the material non-conductive andaccomplishes the isolation described above. This forms the isolationimplant 111 (FIG. 1). The proton implant doses are, in one embodiment asfollows: 2×10¹⁵ at 50 keV, 2×10¹⁵ at 100 keV, 7×10¹⁴ at 161 keV, 7×10¹⁴at 270 keV, and 7×10¹⁴ at 380 keV, at an angle of 7°.

The isolation implant may be used to minimize capacitance. The isolationimplant may be further used to isolate the spokes used to support themetal for the contacts.

Photoresist remaining from the stepper isolation pattern mask is thenremoved. Referring now to FIG. 8A, a metal contact mask defines a regionwhere a top metal contact (p-type intracavity contact 103) will bedeposited on the VCSEL. The metal contact mask results in photoresistbeing on areas where metal is not desired on the VCSEL. A p contact cutis used to expose the top conduction layer 116. The p contact cut maybe, for example, a plasma etch. Metal is then deposited on the entirewafer. A liftoff process is then performed, which causes thephotoresist, and any metal on the photoresist, to be removed from theVCSEL. The remaining metal deposited on the VCSEL becomes the p-typeintracavity contact 103. As mentioned, the p-type intracavity contact103 provides not only an electrical contact for routing current to theactive region, but in the present example, also includes a p-metaloption 312 that provides heatsinking functionality to draw heat awayfrom the active region and to dissipate the heat at the top and sides ofthe undoped top mirror mesa of the VCSEL. In this example, the metal isTi/Au that is 300/14,000 Å thick for both the contact and optional metalon the sidewalls of the mesa structure. The Ti is used as a glue layerfollowed by a thick Au layer.

FIGS. 8B-8E illustrate various other options for metal deposition toform the intracavity contacts 103 and/or 104. For example, FIG. 8Billustrates an example embodiment where the p-type intracavity contact103 does not extend up the mesa structure sidewalls.

FIGS. 8C-8E illustrate examples where a dielectric top mirror is used(such as is set forth in Table 2 attached hereto) as opposed to asemiconductor top mirror 102 (such as is set forth in Table 1 attachedhereto). FIG. 8C. illustrates an example that includes a AuGe n-contactmetal layer 310, a p-contact metal layer 802, and n and p metalcontact/interconnect layers 104, 103. The n and p metalcontact/interconnect layers 104, 103 may be used to connect to the bondpads 902, 904 (FIG. 9) for providing connection points to the VCSEL 100.FIG. 8C further illustrates an interlevel dielectric 804 such as BCBthat may be used to separate the metal contact/interconnect layers 104,103.

FIG. 8D illustrates an example similar to that of 8C which incorporatesan AuGe n-contact metal layer 310, a p-contact metal layer 802, and nand p metal contact/interconnect layers 104, 103 where the n and p metalcontact/interconnect layers 104, 103 may be used to connect to the bondpads 902, 904 (FIG. 9) for providing connection points to the VCSEL 100.FIG. 8D also illustrates an interlevel dielectric 804.

FIG. 8E is also similar to 8D except that FIG. 8E illustrates a VCSELusing a dielectric mirror that omits the p-contact metal layer 802 anduses a p-metal contact/interconnect layer 103.

For reference, FIG. 9 illustrates a completed top view of the VCSEL 100.Specifically the n-type 104 and p-type contacts 103 are shown connectedto other parts of the VCSEL structure previously described herein. FIG.9 illustrates concentric C patterns 906, 908. The concentric C patterns906, 908 allow for contact to the VCSEL 100 to be made without shortingand without intralevel dielectrics. The inner C 908 may be a completecircle. However, using a C pattern allows for a current asymmetry forpolarization control to be implemented. Further, by using the C patternon the inner C 908, processing can be facilitated by allow for lift-offlithograph steps to be used.

Polarization Control

There are a number of methods to pin polarization in the VCSEL includingvarious gratings, asymmetric current injection schemes, asymmetricmechanical strains from the geometry, as well as various non-circularaperture geometries. All of these may also be used to pin polarization.Additionally, growing the VCSEL off-axis can be used for polarizationcontrol as described below. In addition, it is important to use thevarious asymmetries in combination, otherwise the competition betweenthe effects causes a polarization hopping.

In one embodiment, polarization may be accomplished by using amorphoussilicon gratings. Amorphous silicon gratings function by exhibitingdifferent optical thicknesses for different polarizations of light.Light is largely reflected (about 30%) by materials that have an opticalwavelength of nλ/2 where n is an integer. Additionally, light is largelynot reflected when materials have an optical wavelength of nλ/4 where nis an odd integer. High reflectivity lowers the threshold currentrequired for lasing. Thus, if a polarization to be promoted sees theamorphous silicon grating as an integral multiple of λ/2 and apolarization to be inhibited sees the amorphous silicon grating as anodd integral multiple of λ/4, the polarization to be promoted will havea lower threshold current, while the polarization to be inhibited willhave a higher threshold current. If the threshold difference between thepolarization to be promoted and the polarization to be inhibited isgreat enough, a threshold current can be used to bias the VCSEL suchthat the polarization to be promoted is emitted while there is not asufficient amount of threshold current to allow the polarization to beinhibited to be emitted. The amorphous silicon gratings on a VCSELsurface can therefore be fabricated such that a dominant polarizationcaused by the asymmetries in the VCSEL can be promoted through the useof amorphous silicon gratings while the subservient polarization isinhibited or completely blocked through the use of the amorphous silicongratings. Amorphous silicon gratings used for polarization controlinclude a number of protrusions. The protrusions may be comprised ofSiO₂ or SiN.

To form the grating, amorphous silicon is deposited on the VCSELsurface. The amorphous silicon is then etched using electron beamlithography. Electron beam lithography involves depositing a photoresistlayer on a surface to be etched. An electron beam is then used to exposethe photoresist. The electron beam can be controlled in much the sameway as electrons in a conventional television are controlled. Theelectron beam is used to pattern a region that will be etched leavingthe protrusions. A reactive ion etch (RIE) or other appropriate etch isthen used to etch portions of the amorphous silicon to produce thegrating.

The dominant polarization of light emitted from a VCSEL is dependant onvarious factors. Additionally, it may be difficult to accuratelycalculate or predict the number of periods, pitch, and orientation of agrating for pinning polarization of a VCSEL. Thus, it may be useful totest an assortment of period, pitch and orientation combinations todetermine the optimum amorphous silicon grating for use with aparticular VCSEL design.

Other gratings may be used as well. For example, gratings of siliconnitride on a silicon dioxide layer may be used. In this example, thesilicon nitride is pattered with the gratings pattern. The silicondioxide is optionally patterned or partially patterned with the gratingpattern.

By fabricating one or more wafers that include various combinations ofgrating designs, an optimal combination can be realized.

While gratings have been described herein for polarization control, itshould also be noted that embodiments of the invention may alsoimplement birefringent films in the place of the gratings. Birefringentfilms, similar to amorphous silicon gratings, have different opticalthicknesses depending on the polarization of light passing through them.Thus, if a film can be made that is such that a polarization to bepromoted sees the birefringent film as an integral multiple of λ/2 and apolarization to be inhibited sees the birefringent film as an oddintegral multiple of λ/4, the polarization to be promoted will have alower threshold current, while the polarization to be inhibited willhave a higher threshold current. In addition to amorphous silicongratings, gratings of silicon nitride on a silicon dioxide layer forexample can be used.

An inherent or intentional aperture asymmetry may be used in combinationwith a grating to improve VCSEL performance to pin polarization. Forexample, various trench designs can be used to create oval or othershaped apertures.

Other asymmetries may also be used with gratings and birefringent filmsto pin polarization. One asymmetry that may be used to controlpolarization relates to current injection in the VCSEL. Currentinjection refers to the currents that are passed through the VCSEL tostimulate photon emissions in the active region of the VCSEL. Asillustrated in FIG. 9 and discussed previously herein, by forming theinner C 908 as a C pattern allows for a current asymmetry forpolarization control to be implemented.

Another type of asymmetry may be accomplished by fabricating the VCSELin an off-axis orientation. To form the quantum wells in an off-axisorientation, a surfactant such as Sb is used as described above. In thisexample the fabrication is off of the 100 orientation. In oneembodiment, the quantum wells are formed at a 311 orientation, which is29.5° off the 100 orientation. Using Sb and migration enhanced epitaxy,small seeds that would normally be present are suppressed such that theyare small enough sufficient to allow off orientation structures.Migration enhanced epitaxy is described in more detail in U.S. patentapplication Ser. No. 10/931,194 filed on Aug. 31, 2004 and in relatedapplications including application Ser. Nos. 09/217,223 filed Dec. 12,1998; Ser. No. 10/026,016 filed Dec. 20, 2001, Ser. No. 10/026,019 filedDec. 20, 2001, Ser. No. 10/026,044 filed Dec. 27, 2001 and Ser. No.10/026,020 filed Dec. 27, 2001. Each of the cited applications isincorporated herein in their entireties.

By forming the quantum wells in an off-axis orientation, thepolarization can be pinned. This allows optical isolation to beaccomplished by using an inexpensive quarter wave plate. Polarized lightfrom the VCSEL passing through the quarter wave plate and beingreflected back through the quarter wave plate is orthogonal to the lightemitted from the VCSEL. As such, the VCSEL will be insensitive to thisreflected light. Therefore, long wavelength VCSELs can be fabricated forapplications that require polarization stability.

Thermal asymmetries may also be used to facilitate emissions in onepolarization while inhibiting the emissions in an orthogonalpolarization in a VCSEL with integrated photodiode device. This may beaccomplished by forming metal contacts on the device such that heat ismore effectively dissipated from some parts of the device than fromother parts of the device. This results in better VCSEL performance forone polarization of light than for an orthogonal orientation.

Thermal Performance

The removal of heat from the device is important to maximize the peakpower and enhance reliability. The aspects of the epitaxial structurewhich facilitate this are the extensive use of highly thermallyconductive binary compounds close to the quantum wells 112. This isshown in the structure of Table 1 and FIG. 2A where extensive use ofGaAs is made adjacent to the quantum wells 112 in the conduction layer117 below and spacer above 156, just above the oxide with the conductionlayers, and in both mirrors 102, 104. In addition, the lower mirror 104makes extensive use of AlAs which has better thermal conductivity thanGaAs. AlAs is used extensively in the lower mirror 104, but notnecessarily immediately underneath the lower conduction layers 117.Lower aluminum compositions are used adjacent the lower conductionlayers 117 to facilitate process tolerance. The upper mirror 102 canalso be a dielectric mirror in which case its thermal conductivity ispoor and does not contribute much in terms of thermal dissipation.

High temperature performance is limited by a combination of effects. Athigh temperatures confinement of carriers to the quantum wells 112becomes marginal. This of course is helped by good thermal design toremove heat, and minimizing series resistance to avoid excess heatgeneration. In addition, the depth of the quantum wells 112 is importantto reduce carrier leakage past the quantum wells. For InGaAsN(Sb)containing quantum wells, the nitrogen enhances the depth of theconduction band quantum well significantly which reduces electronleakage. Sb on the other hand, increases the valence band well, butdecreases the conduction band well. A reasonable balance may be achievedwith the quantum well compositions presented here. In addition, dopingspikes, and extra AlGaAs confining layers as shown in Tables 1 and 2 canbe used to enhance confinement.

Single Mode Performance

The ability of the VCSEL 100 to remain single mode is important for manyapplications. There are several aspects to a VCSEL design that tend toenhance the ability to remain single mode. To the extent that higherorder modes are supported by the cavity and the gain profile they willexist. To avoid higher order modes the cavity is designed so higherorder modes are not supported, and the gain profile adequately pumps thefundamental mode. A large portion of the lateral optical confinementcomes from thermal lensing which is a direct result of resistive heatingcombined with free carrier absorption, and non radiative recombination.This is minimized with good thermal design, low resistance and low freecarrier absorption. Another way to reduce the ability of the cavity tosupport higher order modes is to make it small laterally, and tallvertically. The large vertical distance allows for mode expansion inregions of lowered thermal lensing (away from the quantum wells 112).The larger sized higher order modes are scattered by the oxide aperture124 (FIG. 1), while the fundamental which is more easily guided is not.To create significant scattering by the oxide aperture 124 it isadvantageous to place it off a null. When it is centered on the null ithas a minimal effect. When it is centered at the peak of the field itprovides excessive guiding itself. The oxide aperture may be placedsomewhere on the order of ⅛th wave below the null in one embodiment.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges that come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A VCSEL comprising: a substrate; an essentiallyundoped bottom DBR mirror disposed on the substrate; a periodicallydoped first conduction layer region of a first conductivity type, thefirst conduction layer region coupled to the bottom DBR mirror andhaving a heavily doped layer at a location where the optical electricfield is at about a minimum; an active layer region disposed on thefirst conduction layer region that contains one or more migrationenhanced epitaxy layer regions before and between N-containing quantumwells each migration enhanced epitaxy layer region having at least oneflattened layer under on a substrate side of a N-containing layer of themigration enhanced epitaxy layer region or a substrate side of theN-containing quantum well, wherein the quantum wells are InGaAsN quantumwells with Sb, wherein the nitrogen concentration is about 2% and theconcentration of the In is less than about 28% and In and Sb together isless than or equal to about 28%; a periodically doped second conductionlayer region of a second conductivity type, the doped second conductionlayer region coupled to the active layer region and having a heavilydoped layer at a location where the optical electric field is at about aminimum; a ramp layer region positioned between the periodically dopedsecond donduction layer region and an aperture, the ramp layer regionincluding a ramp of aluminum in a composition of the ramp layer region;and a top mirror coupled to the second conduction layer region, wherein:the aperture being above the quantum wells and below the periodicallydoped second conduction layer region and being defined by an oxidizedAlGaAs layer; the top mirror is essentially undoped and includes aplurality of layers of AlGaAs having an aluminum content in a range from70% to 100%; and a protective oxide layer surrounding the top mirror toprotect at least a portion of the top mirror from being oxidized duringa wet oxidation step, wherein the heavily doped layer of theperiodically doped second conduction layer region, the ramp layerregion, and the oxidized AlGaAs layer are within a single null of astanding wave of the VCSEL.
 2. The VCSEL of claim 1, wherein the Indium(In) is about 26%.
 3. The VCSEL as recited in claim 1, wherein the VCSELis designed to operate at wavelengths above about 1260 nm.
 4. The VCSELas recited in claim 1, wherein the VCSEL is designed to operate atwavelengths above about 850 nm.
 5. The VCSEL of claim 1, wherein thequantum wells are about In_(0.26)Ga_(0.74)As_(0.96)N_(0.02)Sb_(0.02). 6.The VCSEL of claim 5, wherein the active region includes 4 quantumwells.
 7. The VCSEL of claim 6, comprising barrier layers associatedwith the quantum wells, the barrier layers having N and/or Sb, andhaving one or more of the flattened layers that are devoid of N beforeand/or after and/or between the one or more N-containing barrier layers.8. The VCSEL of claim 1, wherein the one or more migration enhancedepitaxy layer regions include one or more N-containing layers with oneor more of the flattened layers that are devoid of N before and/or afterand/or between the one or more N-containing layers.
 9. The VCSEL ofclaim 8, wherein the one or more migration enhanced epitaxy layerregions include flattened layers formed from alternating single atomiclayers of group III constituents and group V constituents.
 10. The VCSELof claim 9, wherein the single atomic layers are Ga or As.
 11. The VCSELof claim 9, wherein: each quantum well thickness is approximately 50 Åor less; 4 quantum wells are included in the active region; and thequantum wells are about In_(0.26)Ga_(0.74)As_(0.96)N_(0.02)Sb_(0.02).12. The VCSEL of claim 11, substantially devoid of a quantum dot or3-Dimensional growth effects.
 13. The VCSEL of claim 1, wherein eachflattened layer is devoid of N and on a substrate side of anN-containing layer or quantum well.
 14. The VCSEL of claim 1, whereinthe active region comprises N-containing layers interspersed withN-devoid layers associated with the quantum wells or quantum wellbarriers, wherein the N-devoid layers include single atomic layers ofgroup III and group V constituents and are each flattened.
 15. The VCSELof claim 14, wherein the N-containing layers are deposited on N-devoidlayers, the N-containing layers include an amount of In and/or Sb thatcreates a compressive strain between the N-containing layers and theflattened N-devoid layers, without relaxation, thereby decreasing theband gap energy.
 16. The VCSEL of claim 14, wherein the quantum wellsare about In_(0.26)Ga_(0.74)As_(0.96)N_(0.02)Sb_(0.02).